A medium sized, $20$ acre, mixed$-$use commercial development is planned, consisting of offices, retail stores, and residential. The existing frontage road is a nearly level, straight, two$-$lane roadway with unlimited sight distance and no median. The posted speed and average speed are both $45mph.$ Traffic currently operates at LOS C$,$ and there is no signal planned along the proposed frontage. Analysis of site access is being performed to determine design elements. For vehicle access, five common design groups from AASHTO design guides are being used. Passenger car $($P$)$; Single$-$Unit Truck SU$-9($SU$-30)$; Intermediate Semitrailer WB$-12($WB$-40)$; Interstate Semitrailer WB$-20($WB$-67)$; doubletrailer combination WB$-22($WB$-67$D$).$

I. What is the minimum design vehicle for this site? Circle the correct answer.

a$.SU-9(5U-30)$

b$.$ WB $-12($WB$-40)$

c$.$ WB$-20($WB$-67)$

d$.$ WB$-22($WB$-67$D$)$